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Optical scatterometry is one of the most important techniques for measuring the critical dimension (CD) and overlay of nanostructures in current semiconductor manufacturing due to its inherent noncontact, nondestructive, time-effective, and relatively inexpensive merits over other metrology techniques, such as scanning electron microscopy (SEM) and atomic force microscopy. Along with the advantages of optical scatterometry, there are some challenges or limitations to this technique with the ever-decreasing dimensions of advanced technology nodes (22 nm and beyond) [1], such as the parameter correlation issue. In addition, optical scatterometry is mostly suitable for measuring repetitive dense structures while infeasible for the measurement of isolated or the general non-periodic structures. To address the challenges or limitations in conventional optical scatterometry, we have developed a novel instrument called the tomographic Mueller-matrix scatterometer (TMS) [2], which is a combination of a dual rotating-compensator Mueller matrix ellipsometer (MME) [3] and a reflection microscope. In this talk, I will present the development of TMS as well as its application for nanostructure metrology.
As shown in Fig. 1, the position of the focal point of the light beam on the back focal plane (BFP) of a high-numerical-aperture objective lens OL can be changed by rotating the flat mirror FM, which further leads to the change of illumination direction on the sample. An epi-illumination setup is designed to collect the scattered-field distribution associated with each illumination direction by imaging the BFP of the OL. Thanks to the dual rotating-compensator configuration, a 4-by-4 Mueller matrix associated with each point on the BFP of the OL can be obtained. Since the 16 elements of a Mueller matrix contain all polarization information that one can extract from a linear polarization scattering process, the full polarization properties of the scattered field are thus achieved. Details about the principle as well as the calibration of the TMS can be found in Ref. [4, 5] and are omitted here for the sake of brevity.
As a demonstration of the potential of the develop instrument, the TMS was employed for the measurement of a Si grating and a photoresist (PR) grating. For the Si grating, its geometrical profile is characterized by top CD x1, grating height x2, and sidewall angle x3. For the PR grating, its geometrical profile is characterized by top CD x1, grating height x2, sidewall angle x3, and top corner rounding x4. The period of the Si grating is 800 nm, and the nominal dimensions of other structural parameters are x1 = 350 nm, x2 = 470 nm, and x3 = 88 degree, respectively. The period of the photoresist grating is 412 nm, and the nominal dimensions of other structural parameters are x1 = 200 nm, x2 = 311 nm, and x3 = 90 degree, respectively. Figure 2 presents the fitting result of the measured and calculated best-fit Mueller matrices of the Si and PR gratings at different measurement configurations. Good agreement can be observed from this figure for both the two grating samples.
References
[1] N. G. Orji, M. Badaroglu, B. M. Barnes, C. Beitia, B. D. Bunday, U. Celano, R. J. Kline, M. Neisser, Y. Obeng, and A. E. Vladar, Nat. Electron. 1, 532-547 (2018).
[2] Y. Tan, C. Chen, X. Chen, W. Du, H. Gu, and S. Liu, Rev. Sci. Instrum. 89, 073702 (2018).
[3] C. Chen, X. Chen, Y. Shi, H. Gu, H. Jiang, and S. Liu, Appl. Sci. 8, 2583 (2018).
[4] S. Liu, X. Chen, and C. Zhang, Thin Solid Films 584, 176-185 (2015).
[5] C. Chen, X. Chen, H. Gu, H. Jiang, C. Zhang, and S. Liu, Meas. Sci. Technol. 30, 025201 (2019).
(See attached documents for the cited figures).
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